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Fluidelastic Instability (FEI) is the most destructive excitation mechanism in cross-flow heat exchangers. There have been an extensive experimental efforts that were directed at understanding and characterizing this phenomenon.  The bulk of the work was carried out for single phase flow conditions.  Two-phase flow FEI is considered more complex than single-phase FEI due to the addition of the phases interaction.  The existing works focused at correlating the critical flow velocity to the mass damping parameter and void fraction. Several simulated two-phase conditions such as Air-Water, steam-water or refrigerants were utilized. Analytical studies focused mainly on single-phase flow. This study introduces a mechanistic model to simulate the two-phase flow in a normal square bundle to predict the instability threshold. The model was developed considering bubbly flow regime of Air-Water mixture with void fractions up to 40%. 
The model considers the vibrations of a single flexible tube in an otherwise rigid array. The flow around the tube is idealized as one-dimensional flow in channels and is composed of two phases; continuous phase and dispersed phase. The dispersed phase is accounted for by random spherical bubbles in the flow. The motion of each individual bubble was modeled including bubble-to-bubble interaction, break-up, and coalescence. In this framework, the bubbles motion in the channels as well as the bubble compressibility were considered.  By solving the conservation equations, the flow pressure was determined, then integrated to calculate the force on the tube. The stability threshold was calculated through the time-domain simulations of the tube dynamics. The simulation results were compared with experimental data from the literature and a good agreement was obtained.

Fluidelastic Instability, Tube Bundles, Heat Exchangers